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The BMI gives healthcare professionals a consistent way of assessing their patients’ weight and an objective way of discussing it with them. It is also useful in suggesting the degree to which the patient may be at risk for obesity-related diseases.

BMI is a statistical calculation intended as an assessment tool. It can be applied to groups of people to determine trends or it can be applied to individuals. When applied to individuals, it is only one of several assessments used to determine health risks related to being underweight, overweight, or obese.

The history of BMI

The formula used to calculate BMI was developed more than one hundred years ago by Belgian mathematician and scientist Lambert Adolphe Quetelet (1796-1874). Quetelet, who called his calculation the Quetelet Index of Obesity, was one of the first statisticians to apply the concept of a regular bell-shaped statistical distribution to physical and behavioral features of humans. He believed that by careful measurement and statistical analysis, the general characteristics of populations could be mathematically determined. Mathematically describing the traits of a population led him to the concept of the hypothetical “average man” against which other individuals could be measured. In his quest to describe the weight to height relationship in the average man, he developed the formula for calculating the body mass index.

Calculating BMI requires two measurements: weight and height. To calculate BMI using metric units, weight in kilograms (kg) is divided by the height squared measured in meters (m). To calculate BMI in imperial units, weight in pounds (lb) is divided by height squared in inches (in) and then multiplied by 703. This calculation produces a number that is the individual’s BMI This number, when compared to the statistical distribution of BMIs for adults ages 20–29, indicates whether the individual is underweight, average weight, overweight, or obese. The 20–29 age group was chosen as the standard because it represents fully developed adults at the point in their lives when they statistically have the least amount of body fat. The formula for calculating the BMI of children is the same as for adults, but the resulting number is interpreted differently.

Although the formula for calculating BMI was developed in the mid-1800s, it was not commonly used in the United States before the mid-1980s. Until then, fatness or thinness was determined by tables that set an ideal weight or weight range for each height. Heights were measured in one-inch intervals, and the ideal weight range was calculated separately for men and women. The information used to develop these ideal weight-for-height tables came from several decades of data compiled by life insurance companies. These tables determined the probability of death as it related to height and weight and were used by the companies to set life insurance rates. The data excluded anyone with a chronic disease or anyone who, for whatever health reason, could not obtain life insurance.

Interest in using the BMI in the United States increased in the early 1980s when researchers became concerned that Americans were rapidly becoming

KEY TERMS

Morbid obesity—A term used to describe individuals 100 lb (45 kg) or more than 50% overweight and/or who have a body mass index above 40.

Triglycerides—A type of fat found in the blood. High levels of triglycerides can increase the risk of coronary artery disease.

obese. In 1984 the national percentage of overweight individuals was reported in a major assessment of the nation’s health. Men having a BMI of 28 or greater were considered overweight. This BMInumber was chosen to define overweight because 85% of American men ages 20-29 fell below it. A different calculation, not BMI, was used for women in the report.

In 1985, the term overweight was redefined as a BMI equal to or greater than 27.8 for men and equal to or greater than 27.3 for women. No BMI was selected to define underweight individuals. This definition of overweight was used in reports on obesity until 1998. In 1998, the United States National Institutes of Health revised its weight definitions to bring them in line with the definitions used by the World Health Organization. Overnight 30 million Americans went from being classified as normal weight to being classified as overweight. Overweight is now defined for both men and women as a BMI of 25 or less. At the same time, an underweight classification was added, as was the classification of obese for individuals with a BMI greater than or equal to 30.

Interpreting BMI calculations for adults

All adults age 20 and older are evaluated on the same BMI scale as follows:

BMI below 18.5: Underweight

BMI 18.5-24.9: Normal weight

BMI 25.0-29.9: Overweight

BMI 30 and above: Obese

Some researchers consider a BMI of 17 or below an indication of serious, health-threatening malnourishment. In developed countries, a BMI this low in the absence of disease is often an indication anorexia nervosa At the other end of the scale, a BMI of 40 or greater indicates morbid obesity that carries a very high risk of developing obesity-related diseases such as stroke, heart attack, and type 2 diabetes.

Interpreting BMI calculations for children and teens

The formula for calculating the BMI of children ages 2-20 is the same as the formula used in calculating adult BMIs, but the results are interpreted differently. Interpretation of BMI for children takes into consideration that the amount of body fat changes as children grow and that the amount of body fat is different in boys and girls of the same age and weight.

Instead of assigning a child to a specific weight category based on their BMI, a child’s BMI is compared to other children of the same age and sex. Children are then assigned a percentile based on their BMI The percentile provides a comparison between their weight and that of other children the same age and gender. For example, if a girl is in the 75th percentile for her age group, 75 of every 100 children who are her age weigh less than she does and 25 of every 100 weigh more than she does. The weight categories for children are:

Below the 5th percentile: Underweight

5th percentile to less than the 85th percentile: Healthy weight

85th percentile to less than the 95th percentile: At risk of overweight

95th percentile and above: Overweight

Application of BMI information

The BMI was originally designed to observe groups of people. It is still used to spot trends, such as increasing weight in a particular age group over time. It is also a valuable tool for comparing body mass among different ethnic or cultural groups, and can indicate to what degree populations are undernourished or overnourished.

When applied to individuals, the BMI is not a diagnostic tool. Although there is an established link between BMI and the prevalence of certain diseases such as type 2 diabetes, some cancers, and cardiovascular disease, BMI alone is not intended to predict the likelihood of an individual developing these diseases. The National Heart, Lung, and Blood Institute recommends that the following measures be used to assess the impact of weight on health:

BMI is very accurate when defining characteristics of populations, but less accurate when applied to individuals. However, because it is inexpensive and easy to determine BMI is widely used. Calculating BMI requires a scale, a measuring rod, and the ability to do simple arithmetic or use a calculator. Potential limitations of BMI when applied to individuals are:

BMI does not distinguish between fat and muscle. BMI tends to overestimate the degree of “fatness” among elite athletes in sports such as football, weightlifting, and bodybuilding. Since muscle weighs more than fat, many athletes who develop heavily muscled bodies are classified as overweight, even though they have a low percentage of body fat and are in top physical condition.

BMI tends to underestimate the degree of fatness in the elderly as muscle and bone mass is lost and replaced by fat for the same reason it overestimates fatness in athletes.

BMI makes no distinction between body types. People with large frames (big boned) are held to the same standards as people with small frames.

BMI weight classes have absolute cut-offs, while in many cases health risks change gradually along with changing BMIs. A person with a BMI of 24.9 is classified as normal weight, while one with a BMI of 25.1 is overweight. In reality, their health risks may be quite similar.

BMI does not take into consideration diseases or drugs that may cause significant water retention.

BMI makes no distinction between genders, races, or ethnicities. Two people with the same BMI may have different health risks because of their gender or genetic heritage.

BMI is a comparative index and does not measure the amount of body fat directly. Other methods do give a direct measure of body fat, but these methods generally are expensive and require specialized equipment and training to be performed accurately. Among them are measurement of skin fold thickness, underwater (hydrostatic) weighing, bioelectrical impedance, and dual-energy x-ray absorptiometry (DXA). Combining BMI, waist circumference, family health history, and lifestyle analysis gives healthcare providers enough information to analyze health risks related to weight at minimal cost to the patient.

Childhood obesity is an increasing concern. Research shows that overweight children are more likely to become obese adults than normal weight children. Excess weight in childhood is also linked to early development of type 2 diabetes, cardiovascular disease, and early onset of certain cancers. In addition, overweight or severely underweight children often pay a heavy social and emotional price as objects of scorn or teasing.

Both the American Academy of Pediatrics (AAP) and the United States Centers for Disease Control and Prevention (CDC) recommend that the BMI of children over age two be reviewed at regular intervals during pediatric visits. Parents of children whose BMI falls above the 85th percentile (at risk of being overweight and overweight categories) should seek information from their healthcare provider about health risks related to a high BMI and guidance on how to moderate their child’s weight. Strenuous dieting is rarely advised for growing children, but healthcare providers can give guidance on improving the chid’s diet, eliminating empty calories (such as those found in soda and candy) and increasing the child’s activity level in order to burn more calories and improve fitness.